The occlusion effect denotes the low frequency enhancement in the loudness level of bone conducted signals due to occlusion of the ear canal. Some users having an earmold or a hearing aid in the ear canal blocking the canal complain that they have a perception of being in a barrel. In particular, their own voice sounds as if they speak in a barrel.
FIG. 1 shows plots of sound pressure level (SPL) in the ear canal as a function of frequency for a sound with a specific frequency spectrum. SPL is the quantity of sound energy relative to a reference pressure: 20 μPa. The plotted SPL is measured in two situations. Curve 1 shows SPL measured in the occluded ear canal, and curve 2 shows SPL measured in the non-occluded ear canal. It is shown that for low frequencies, the SPL is approximately 10–30 dB higher for an occluded ear canal than for a non-occluded ear canal. The plotted curves are adopted from “The hollow voice occlusion effect”, M. Killion, FIG. 6, “Hearing aid fitting”, J. Jensen, p. 231, 13'th Danavox Symposium, 1988.
Sounds produced in a person's throat are transmitted to the person's ear canal by bone conduction. The elastic cartilaginous tissue in the ear canal transforms the bone conducted energy to acoustic waves in the ear canal. Speech transmitted to the ear canal in this way is denoted bone conducted speech.
It is known to suppress the occlusion effect by inserting the hearing aid earmold or housing deeply in the ear canal, i.e. in the bony part of the ear canal. This reduces the occlusion effect since the sealed volume of the ear canal is isolated from the cartilaginous tissue transforming bone conducted speech to acoustic waves. However, the bony part of the ear canal is typically very sensitive and positioning of a mechanical member in this part of the ear canal may be uncomfortable to the user.
It is also well known to provide a vent in the earmold or hearing aid housing allowing bone conducted sound to escape from the ear canal. The vent is typically a tube extending through the earmold or hearing aid housing facilitating transmission of acoustic waves from one side to the other so that the ear canal is not completely blocked. However, the vent may cause acoustic feedback. Acoustic feedback occurs when the microphone of a hearing aid receives the acoustic output signal generated by the receiver. Amplification of the received signal may lead to generation of a stronger acoustic output signal and eventually the hearing aid may oscillate. In hearing aids residing completely in the canal (CIC hearing aids), the short distance between microphone and receiver leads to low attenuation of acoustic waves transmitted from the receiver to the microphone. The attenuation increases with decreasing vent diameter and increasing vent length. Thus, occlusion and feedback impose opposite requirements on vent geometry.